The 6-halo-2-chloroquinoxaline compounds are starting reagents for the preparation of quinoxalinyloxy ether compounds [European Patent Office Application No. 81302801.6, published Dec. 30, 1981] which have utility as herbicides, particularly for the control of grass weeds in paddy rice, soybeans, cotton, potatoes, etc. Rice is the second largest food crop with over 320 million metric tons annual production worldwide [J. R. Harlan, 235 Sci. Amer., 88 (1976)]. Undesired competing vegetation, such as the extremely prevalent barnyardgrass (Echinochloa crusgalli) and closely related species can reduce rice yield by 25% if allowed to compete all season at an infestation of only one plant per square foot [Agricultural Age, May 1977, page 11]. Soybeans, with over 60 million metric tons annual production worldwide [Sci. Amer., 235, 88 (1976)] is an extremely valuable crop which must often compete with such grassweeds as crabgrass, barnyardgrass, and wild oats. By inhibiting the growth or killing such undesired vegetation, significant improvements in agricultural efficiency are realized.
Toman et al. [Coll. Czech. Chem. Commun., 43, 2179 (1978)] disclose a procedure for reaction of 4-chloro-2-nitroaniline with diketene in refluxing chlorobenzene at 132.degree. C. The use of such high temperatures results in competing decomposition and polymerization reactions of diketene and a lower yield. Toman et al. also disclose a procedure for reaction of 4-chloro-2-nitroacetoacetanilide with base in aqueous ethanol to produce the 2-hydroxyquinoxaline-4-oxide. The use of ethanol-water solvent is costly, precludes direct continued use of the reaction product in subsequent processing steps and results in waste streams containing flammable materials.
U.S. Pat. No. 3,708,580 discloses the reaction of substituted 2-nitroanilines with diketene in a solution of glacial acetic acid and mercuric acetate. Continued processing of the 2-nitroacetoacetanilide requires additional separation steps. Also disclosed is the reaction of substituted 2-nitroacetoacetanilides with 18% potassium hydroxide.
Lacey [J. Chem. Soc., 850 (1954)] discloses the use of triethylamine as a catalyst for reactions of diketene with alkoxycarbonyl-amines to produce derivatives of acetoacetamide. The aromatic amine compounds studied do not, however, include either nitro- or halogen substitution, which as electron withdrawing substituents, reduce the basicity of the amine group. Such reduced basicity is stated by Lacey to result in sluggish reaction.
Tennant [J. Chem. Soc., 1963, 2428] describes the synthesis of unsubstituted 2-hydroxyquinoxaline-4-oxide, and also its reduction with sodium dithionite in acetic acid to produce 2-quinoxalinol. The yield was not reported, and the reduction of compounds with halogen substitution at the six (6)-position was not considered.
Amad et al. [Tetrahedron 20, 1107 (1964)] reports the use of sodium dithionite to reduce 6-chloro-3-cyano-2-hydroxyquinoxaline-4-oxide. The reaction results in the loss of the nitrile group (strongly electron withdrawing) with simultaneous formation of 2-hydroxyquinoxaline.
Katritzky and Monro [J. Chem. Soc., 1958, 1263] describe the reduction of substituted pyridine N-oxides using hydrogen and palladium/carbon catalyst. The substituted fused-ring quinoline 4-oxides are however reported to be resistant to such reduction.
Taylor and Jefford [Chem. and Ind., 1963, 1559] disclose the reduction of quinoxaline-4-oxides with amino substitution to block the reactive three (3)-position using hydrogen and Raney nickel catalyst. The reaction is run in a mixture of ethanol and methylamine, and the effect of halogen substitution in the aromatic ring is not considered.
Crowther et al. [J. Chem. Soc., 1949, 1260] discloses the use of phosphorus oxychloride for the conversion of 2-hydroxyquinoxalines to 2-chloroquinoxalines. The phosphorous compound by-products are removed by reacting with ice-water to decompose them to an aqueous phosphoric acid solution.
Given the anticipated importance of quinoxalinyloxy ether herbicides, what is needed is an economical process for producing 6-halo-2-chloroquinoxaline compounds at high yield without contamination by isomeric impurities.